In the directional drilling of oil well boreholes, it is not uncommon for the drillstring to become mechanically stuck within the borehole. The recovery and replacement cost associated with a stuck drillstring are very high. Accordingly, there is an interest in the art in developing methods for minimizing such recovery and replacement costs. Determining the free point of a stuck drillstring allows removal of the maximum possible quantity of drillstring from the wellbore on the first pass and thus reduction in the number of passes necessary for the attempted removal of the remaining drillstring.
There are two conventional methods for determining the Point in the drillstring below which the string is stuck and above which the string is free.
The Brouse method involves applying excess tension to the drillstring, i.e. overpull, and measuring the resultant stretch of the drillpipe. Using the measurements of applied force and stretch in view of a simple mechanical relationship determines the length of pipe above the free point. The method assumes that there exists a length of drillpipe of uniform physical dimensions and properties between the surface and the free point. Several problems are associated with the Brouse method. It is necessary to apply additional tensile loading to the drillstring to induce stretching of the pipe.
There exists a risk that depending upon the condition of the pipe, the drillstring could be torn apart by the application of the excess force. The problem becomes more serious in situations where various sizes of pipe, made from different materials, are used in a tapering configuration, e.g. as in horizontal drilling. The method is reasonably accurate when the drillstring between the stuck point and the surface consists of a number of sections of drillpipe each having similar physical properties and dimensions. The Brouse method becomes increasingly inaccurate with increasing drillstring complexity. Finally, the Brouse method is unable to estimate the free point if it occurs within the bottom hole assembly.
A wire line tool may also be used to determine the free Point. The wire line tool method requires that a tool be run inside the drillstring on an electric line. The tool is positioned near the estimated free point. The tool has means to fix its position relative to the drillstring. With the tool in place, tensile and rotational forces are applied to the drillstring. The relative movement of the tool ends is measured, and the location of the free point is determined from these measurements. Several problems are associated with the wire line tool method of free point determination. The drillstring is subjected to excess tensile and torsional forces which increase the risk of failure of the drillpipe as discussed above. The wireline tool method is very time consuming and expensive requiring the use of an electric wireline and qualified operators. The method is inherently unsafe due to the presence of the wireline in the drillpipe and requires elaborate precautions against a well blow out. The method is generally accurate in most situations.
What is needed in the art, is a method for determining the free point of a stuck drillstring which overcomes the above deficiencies.